I had an low configuration comp but now I changed it with a new 1 that is AMD PHENON 2 and NVIDEA 1gb graphic card and 4GB RAM but the problem is that my old monitor(SAMSUNG SAMTON 454Bn) is not displaying any thing is showing blank screen... what can I do so that I can make it work rether buying a new moitor..

Verify that the connection is secure.
Make certain that you have the monitor plugged in and turned on.
If your monitor has multiple inputs, ensure you are on the correct input (some monitors have both VGA and DVI inputs, and others will even have HDMI connections).
If your graphics card has more than one output, try the other output. Usually there is a 1 and a 2 next to the two outputs on the cards.

If you are still having problems, you could also try connecting to the graphics card built into your motherboard (if it has one). It could be that the external card will not work until you disable the built-in graphics card in your BIOS.